From statistics taken from the American Heart Association, it is known that over 40 million Americans have some form of heart and/or blood vessel disease. Over one million deaths occur annually due to cardiovascular disease, and over 600,000 deaths are the result of coronary artery disease. Accurate diagnosis and appropriate therapy are critical to the management of a patient with cardiovascular disease.
Many diagnostic tools are available to diagnose coronary artery disease or heart attack. These include blood tests, electrocardiograms (resting or during stress), angiography (conventional and digital subtraction techniques), ultrasound and nuclear cardiology techniques. The nuclear cardiology techniques, which employ nuclear imaging, are the only techniques capable of functional assessment of the heart. Nuclear cardiology techniques are capable of detecting infractions, ischemia, coronary artery disease, assessment of birth defects and predicting effectiveness of cardiac medications and/or surgical intervention.
Relative to other diagnostic imaging techniques, nuclear imaging has several important advantages which account for its current growth. Most important, nuclear imaging can provide diagnostic information related to cardiac function rather than just anatomy. By utilizing radioactive tracers, nuclear imaging of left ventricular function (LVF) can monitor physiological processes over time, whereas most other imaging methods can produce only a static picture. Therefore, the use of radionuclides in diagnosis of cardiovascular disease is continually expanding.
In addition to the diagnostic imaging procedure, an important need exists for a device which permits nuclear and ECG measurements to be made in an ambulatory mode. This need exists because, during the performance of ordinary activities associated with daily living, left ventricular function varies over a wide range in both the healthy and diseased heart. These changes in left ventricular function, brought about by such ordinary activities as walking, climbing stairs, psychological stress, exposure to severe temperature changes, etc., may equal or exceed those observed in a laboratory during the performance of a nuclear cardiac dynamic function study. In coronary artery disease, the accurate and continuous measurement of changes in cardiac physiology such as ischemia, arrhythmia, fall in ejection fraction, or a rise in relative cardiac blood volume can assist in the management of the patient's disease. In addition, measurements made before and after surgery or drug therapy may offer additional insights into the impact of these treatments on left ventricular function or dysfunction.
Likewise, in silent ischemia (also defined by many cardiologists as left ventricular dysfunction), where electrocardiographic changes may possibly be observed after several minutes of ECG recording, left ventricular function changes may be observed in a matter of seconds after the onset of the decompensation. The effective monitoring of these left ventricular changes (such as, increase in end systolic volume) result in better design and administration of a proper therapy regime.
An example of a nuclear cardiac probe designed to meet the need for noninvasive evaluation of rapidly developing flanges in global left ventricular function is discussed in "The Nuclear Cardiac Probe," by Dr. Henry N. Wagner Jr., Hospital Practice, April 1982, Volume 17, Number 4, pages 163-177. The probe discussed in the article is housed in a console which may be moved by casters from place to place. The probe, however, does not offer a system that can be easily carried by the patient.
Ambulatory monitoring of left ventricular function has been shown to be possible with the development of a miniaturized system of radionuclide detectors and electronics incorporated into a vestlike garment and worn outside the chest. See, for example, "An Ambulatory Ventricular Function Monitor: Validation and Preliminary Clinical Results," by Drs. Wilson, Sullivan, Moore, Zielonka, Alpert, Boucher, McKusick and Strauss, The American Journal of Cardiology, Sept. 1, 1983, Volume 52, pages 601-606.
A truly ambulatory cardiac evaluation system has several potential areas of application. Firstly, it may be particularly useful in evaluating the incidence of silent ischemia. There is now tremendous interest in the cardiology community in the idea that many of the episodes of myocardial ischemia in patients with coronary disease are probably pain free. There has been much talk that ST segment changes seen on Holter recordings may represent ischemia. That, however, has been extremely controversial because people are aware of other circumstances where ST segment changes are not caused by ischemia. Therefore, the issue has been to identify changes in ventricular function which could be caused by ischemia in association with the ST changes. This has been something which is very difficult to identify in ambulatory subjects. The present invention can make these measurements at the same time.
The second application is to define the impact of drug therapy. This is particularly important in patients who have just been diagnosed as having coronary disease, hypertension or some other circumstances where there is a need to know whether the drug therapy has depressed the patients ventricular function. The patient can be studied before and after taking the drug. In both cases, the patient pursues his/her daily activities to see whether the drug has negatively impacted cardiac function. Currently this is done by merely monitoring the patients reaction--do they feel tired, get out of breath, etc.
The third area is to define the appropriate exercise prescription in both people who do not have known heart disease, but are just out of shape, and in people who have known heart disease. It is particularly useful on patients after they have had a myocardial infarction where the patient should begin exercising on a gradual basis so that they do not exercise to a point where their ventricular function diminishes.
Thus, there is still a need for an ambulatory evaluation system which can be worn in relative comfort by a patient for monitoring coronary artery disease, in surgical and post-operative workups, for anesthesia rehabilitation, for monitoring exercise regime, for drug and diet studies, and for monitoring the effectiveness of drug administered in the therapeutic program. The present invention is directed toward filling that need.